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Quarterly Journal of the Royal Meteorological Society
Q. J. R. Meteorol. Soc. (2014) DOI:10.1002/qj.2416
Impact of sub-pixel rainfall variability on spaceborne precipitation
estimation: evaluating the TRMM 2A25 product
Pierre-Emmanuel Kirstetter,a,b,c Y. Hong,a,c * J. J. Gourley,b M. Schwaller,d
W. Petersene and Qing Caoc
a
School of Civil Engineering and Environmental Sciences, University of Oklahoma, OK, USA
b
NOAA/National Severe Storms Laboratory, Norman, OK, USA
c
Advanced Radar Research Center, National Weather Center, Norman, OK, USA
d
NASA Goddard Space Flight Center, Greenbelt, MD, USA
e
NASA Wallops Flight Facility, Wallops Island, VA, USA
*Correspondence to: Y. Hong, National Weather Center Advanced Radar Research Center 4610, 120 David L. Boren Blvd, Norman,
OK 73072-7303, USA. E-mail: yanghong@ou.edu
Rain intensity spectra as seen by space sensors feed numerous applications at global scales
ranging from water budget studies to forecasting natural hazards related to extreme rainfall
events. Rainfall variability at scales finer than what is resolved by current space sensors affects
their detection capabilities, the characterization of rainfall types, as well as the quantification
of rainfall rates. A high-resolution surface rainfall product is used to evaluate the impact
of rainfall variability within the field of view (FOV) of the Tropical Rainfall Measurement
Mission (TRMM) Precipitation Radar (PR) quantitative precipitation estimation (QPE) at
ground. The primary contribution of this study is to assess the impact of rainfall variability
in terms of occurrence, types and rate at PR’s pixel resolution on PR precipitation detection,
classification and quantification. Several aspects of PR errors are revealed and quantified
including sensitivity to non-uniform beam filling. While the error structure of the PR is
complicated because of the interaction of these factors, simple error models are developed to
describe the PR performances. The methodology and framework developed herein applies
more generally to rainfall rate estimates from other sensors on board low Earth-orbiting
satellites such as microwave imagers and dual-frequency radars such as with the Global
Precipitation Measurement (GPM) mission.
Key Words:
satellite-based rain estimation; radar; QPE; conditional bias; random error
Received 23 May 2013; Revised 19 March 2014; Accepted 1 July 2014; Published online in Wiley Online Library
1. Introduction
In the context of changing climate, rainfall occurrence, types and
intensity are becoming widely evaluated because of their impact
on the energy budget and the hydrological cycle. At the global
scale, only meteorological satellites provide remote-sensing
observations of rainfall, essential for hydrologic and climatic
applications, which range from climatic analysis (Stephens and
Kummerow, 2007), budgeting water resources over land (Grimes
and Diop, 2003; Lebel et al., 2009), real-time flood forecasting
(Hong et al., 2007) to data assimilation and evaluation of
regional and global atmospheric model simulations (Stephens
and Kummerow, 2007). Rainfall fields have variability across a
variety of spatial and temporal scales. Regarding measurement
from remote-sensing platforms, it is important to quantitatively
evaluate the following characteristics of the rainfall rate distribution at the instruments’ pixel scale: occurrence and proportion of
positive values, mean value, variability and types (Figure 1). The
spatial heterogeneity of the rain fields within a single instrumental
c 2014 Royal Meteorological Society
field of view (e.g. beam filling, coexistence of convective and
stratiform precipitation, and vertical heterogeneity of rainfall)
interact with the sensitivity of the instrument itself, the indirect
nature of the measurement, the spatial resolution and the
retrieval algorithm used. As a consequence, satellite surface rain
rate retrievals are not a simple convolution of the fine-scale
rain rate distribution within the field of view of the instrument.
Characterizing the error structure of satellite rainfall products
is recognized as a major issue for the usefulness of the estimates
(Yang et al., 2006; Turk et al., 2008; Sapiano and Arkin, 2009;
Wolff and Fisher, 2009). We address these questions by evaluating
how satellite surface rainfall characteristics, i.e. occurrence, type
and rate, differ from a reference rainfall within the field of view.
These questions address the detection, classification and quantification capabilities of such a radar sensor on board a satellite.
In fact, rainfall variability at scales finer than the typical field of
view of the sensors is recognized as a major source of uncertainty
for rainfall estimation from space (e.g. Iguchi et al., 2009). As an
example regarding rainfall detection, Lin and Hou (2012) showed
P.-E. Kirstetter et al.
Figure 1. (a) Overpass of TRMM-PR over the conterminous U.S. at 0725 UTC on 11 April 2011. NMQ/Q2 instantaneous precipitation rates are shown; (b) zoomed
area showing the variability and spatial distribution of precipitation within TRMM-PR footprints figured with circles; (c) schematic showing the variability can be
addressed with the higher horizontal resolution of NMQ.
how the varying detection capabilities of space-based active and
passive sensors can impact the contribution of different rain
intensity categories to the total rain incidence and rain volume.
We focus primarily on the Tropical Rainfall Measurement
Mission (TRMM) Precipitation Radar (PR) quantitative
precipitation estimation (QPE) at ground. Iguchi et al. (2009)
mentioned how difficult the non-uniform beam-filling issue is
to handle in converting the PR signal into a rain rate. While
the methodology presented herein would apply to all satellite
precipitation products, the TRMM PR is often considered as a
calibrator for other space-based passive microwave sensors such
as the TRMM Microwave Imager (e.g. Yang et al., 2006; Wolff
and Fisher, 2008, 2009). The combination of these sensors in
a constellation collectively enables the creation of global-scale
precipitation products (Ushio et al., 2006; Ebert et al., 2007;
Huffman et al., 2007) and constitutes the backbone of the future
Global Precipitation Measurement (GPM) mission. A number of
studies have investigated the quality of PR estimates in various
regions of the world (e.g. Adeyewa and Nakamura, 2003; Lin and
Hou, 2008; Michaelides, 2008; Wolff and Fisher, 2008, 2009).
Over the USA, Amitai et al. (2009, 2012) have compared the
PR with the National Oceanic and Atmospheric Administration/National Severe Storms Laboratory (NOAA/NSSL) ground
radar-based National Mosaic and QPE system (NMQ/Q2), which
offers a robust set of resources for validation.
In this study, the PR QPE product is assessed over the
southern conterminous USA (CONUS) with respect to the
high-resolution, independent NMQ/Q2 rainfall dataset (Zhang
et al., 2011a) following the methodology and framework for
evaluating PR 2A25 products described in Kirstetter et al. (2012,
2013). This study is part of an effort to perform a systematic
and comprehensive evaluation of PR errors by matching quasiinstantaneous data from Q2 to the ∼5 km pixel measurement scale
of PR in order to minimize uncertainties caused by resampling.
Here, the finer spatial resolution of NMQ (∼1 km) is specifically
used to characterize the rainfall sensed by the PR in terms
of occurrence, type and rate. By doing so, we will also assess
the impact of the small-scale variability of rainfall on the PR
estimates. We evaluate the distribution or spectrum of rainfall
occurrence, typology and quantity within the PR field of view
(FOV) and relate these characteristics to PR rain estimates. This
study used seven months (March–October 2011) of satellite
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overpasses over the lower CONUS (up to latitude 36◦ N). Despite
the seemingly short period for evaluation, the use of gridded Q2
data for reference provided a large sample size totalling 1 625 942
non-zero PR–reference pairs.
The PR data and steps required to refine the Q2 ground-based
rainfall to arrive at the reference rainfall used for comparisons
are presented in section 2. Section 3 assesses the ability of PR rain
retrievals to detect rainfall. Section 4 details the impact of the
beam filling and rain typology on the PR rainfall classification.
Section 5 addresses the aspect of rainfall quantification. The article
is closed with concluding remarks in section 6.
2. Data sources
2.1.
Q2-based reference rainfall and derived products
All rain fields observed coincidentally by TRMM overpasses
and the Next-Generation Weather Doppler Radar (NEXRAD)
radar network from March to October 2011 are collected. The
NMQ-Q2 products closest in time to the TRMM satellite local
overpass schedule time are used. The NOAA/NSSL National
Mosaic and Quantitative Precipitation Estimation system (NMQ:
http://nmq.ou.edu; Zhang et al., 2011a) is a set of experimental
radar products including high-resolution (0.01◦ , 5 min) instantaneous rainfall-rate mosaics available over the CONUS. The
NMQ system gathers information from all ground-based radars
comprising the Weather Surveillance Radar–1988 Doppler
(WSR-88D) network, mosaics reflectivity data onto a common
three-dimensional (3D) grid, and estimates surface rainfall accumulations and types to deliver accurate ground-based estimates
of rainfall (Zhang et al., 2005; Vasiloff et al., 2007). At hourly
time steps, Q2 adjusts radar estimates with automated rain-gauge
networks using a spatially variable bias multiplicative factor. A
radar quality index (RQI) is produced to represent the radar QPE
uncertainty associated with reflectivity changes with height and
near the melting layer (Zhang et al., 2011b). One should note
that it is not possible to ‘validate’ the PR estimates in a strict
sense because independent rainfall estimates with no uncertainty
do not exist. A number of complicating factors impact the radar
QPE uncertainty, such as measurement errors, non-precipitation
echoes, uncertainties in Z –R relationships, and variability in the
Q. J. R. Meteorol. Soc. (2014)
Impact of Sub-Pixel Rainfall Variability on TRMM 2A25
vertical profile of reflectivity associated with range. Trustworthy
values of the Q2 rainfall estimates within the satellite pixel are
needed to evaluate the satellite estimates. To mitigate potential
error sources affecting quantitative precipitation estimation from
ground-based radars and to refine the reference dataset as much
as possible, co-located rain-gauge observations were used to
adjust instantaneous Q2 products (Kirstetter et al., 2012, 2013).
The reference rainfall is derived from an instantaneous gauge
bias-corrected Q2 product adjusted using a spatially variable
multiplicative bias field. A conservative approach is followed:
(i) by filtering out instances when the radar and gauge have significant quantitative disagreement (i.e. radar–rain-gauge ratios
outside of the range (0.1–10)), and (ii) by retaining only the best
measurement conditions (i.e. no beam blockage and radar beam
below the melting layer) using the RQI product as described
in Kirstetter et al. (2012, 2013). One must keep in mind these
improvements may not screen out all possible errors in groundbased radar estimates. Nevertheless, Chen et al. (2013) recently
quantified the errors of Q2 rainfall estimates and provided
uncertainty estimates of hourly rainfall products. They found
that the errors depended strongly on RQI, with there being very
little bias at RQI values of 1, the same threshold used in this study.
A simplified rain type classification was elaborated from the
Vertically Integrated Liquid Content (VIL) derived from the Q2
3D mosaics at the original resolution (1 km, 5 min). A twostep approach similar to Steiner et al. (1995) was applied to
identify convective areas. First the centres of convective cells are
identified from the VIL map using a threshold value (5.5 kg m−2 )
above which it is assumed that precipitation can only result
from convective processes. Then an associated convective region
is identified from surrounding pixels with VIL values greater
than 2 kg m−2 at distances within 20 km. Pixels flagged as
non-convective are designated as stratiform.
Radar observations enable a reliable evaluation of areaaveraged rainfall. The reference rainfall Rref is computed from
a block-Q2 rainfall pixel matching each PR pixel. All of the
Q2 pixels (rainy and non-rainy) found within an approximate
2.5 km radius around the centre of the PR pixel location are
considered to compute unconditional mean rain rates for the
Q2 at the PR pixel scale. The number of Q2 pixels within a
PR footprint used to derive the statistical characteristics of the
reference rainfall tends to average about 25. Additional reference
rainfall characteristics on occurrence, variability and types are
derived within the FOV. Kirstetter et al. (2012) assessed the
estimation reference representativeness using a standard error
computed alongside the mean reference rainfall value: σfootprint ,
quantifying the variability of the Q2 rainfall (at its native 1 km
resolution) inside the PR footprint. It was used to select the
PR–Q2 reference pairs for which the Rref is trustworthy by
segregating the reference pixels into ‘robust’ (Rref > σfootprint )
and ‘non-robust’ (Rref < σfootprint ) estimators (see Kirstetter
et al. (2012) for more details). Later, Kirstetter et al. (2013)
used σfootprint to assess the Non-Uniform Beam Filling (NUBF)
by quantifying the variability of Q2 precipitation within the PR
footprint. In the present study, an extended description of the
reference rainfall within the PR FOV is assessed through three
products derived at the PR footprint resolution:
(i) the rainy fraction (RF) characterizes the rainfall occurrence
within the PR pixel. It represents the FOV filling with the
proportion of positive Q2 values inside the PR pixel and is
expressed in per cent between 0% (no rainfall within the
FOV) and 100% (FOV filled);
(ii) the quantitative variability of the Q2 values. Because
σfootprint presents some correlation with the reference
rainfall rate (Ciach and Krajewski, 2006), we use the
Relative Non-Uniform Beam Filling quantity as RNUBF
σ
= footprint
(unitless) to assess the impact of the variability
Rref
of the Q2 rain rate relative to Rref ; typical values range from
0.1 (homogenous Q2 values within the FOV) to 50 (highly
variable Q2 values);
c 2014 Royal Meteorological Society
(iii) the rainfall type through a Convective Percentage Index
(CPI) quantifying the volume contribution of convective
rainfall to Rref as follows:
nconv
j=1 ωj Q2(aj )
f 2 (θ , θ0 )dθ ,
CPI(A) = 100 n
with ωi =
i=1 ωi Q2(ai )
θmesh (ai )
(1)
where notations, consistent with Kirstetter et al. (2012), have
been simplified for the sake of convenience. Q2 denotes the Q2
rain rate at the original data product resolution (1 km2 ) for the
mesh ai ; n is the number of Q2 data points inside the PR pixel
A; nconv ≤ n is the number of Q2 pixels flagged as convective
inside the pixel A; the weights ω are derived from the two-way
normalized power-gain function of the PR antenna f (assumed to
be Gaussian) and the beam width θ0 ; each ωi is computed over the
domain θmesh corresponding to the Q2 mesh ai . It is assumed the
PR resolution remains constant (circle of 5.1 km) whatever the
radar beam off-nadir inclination angle. Additional research may
be needed to take into account the deformation of the resolution
with off-nadir angle (Takahashi et al., 2006). The CPI is expressed
in per cent between 0% (purely stratiform rainfall within the PR
FOV) to 100% (purely convective rainfall).
2.2.
Precipitation radar (PR)-based rainfall
The PR measures reflectivity profiles in the Ku band. Surface rain
rates are estimated over the southern USA up to a latitude of
36◦ N (see Figure 1, Kirstetter et al., 2012). The scan geometry and
sampling rate of the PR yield footprints spaced approximately
5.1 km in the horizontal and along-track, over a 245 km wide
swath. The TRMM product used in this work is the PR 2A25
product (versions 7) described in Iguchi et al. (2000, 2009) which
provides 3D reflectivity and 2D rain rate fields at ground. The
variable eSurfRain (estimated surface rain) was extracted from
the 2A25 files as the PR surface QPE. The PR detection limit of
rainfall depends primarily on the instrument sensitivity around
17 dBZ. The various conditions of filling of the FOV by rainfall
may impact the detection capabilities of the PR. The 2A23 product
classifies rain into stratiform and convective (Awaka et al., 2007).
Our dataset includes 468 921 pixels classified as stratiform by
the PR and 185 860 convective pixels. The 2A25 algorithm
relies on a hybrid attenuation correction method that combines
the surface reference technique and Hitschfeld–Bordan method
(Iguchi et al., 2000; Meneghini et al., 2000, 2004; Takahashi et al.,
2006). It uses models to describe the hydrometeor drop size
distributions (DSDs), which are adjusted to match the observed
Path Integrated Attenuation. The PR is a well-calibrated and
very stable radar. Primary errors in rainfall retrievals have
mainly been attributed to attenuation correction of the radar
signal (involving the incorrect physical assumptions related to
convective versus stratiform rainfall classification and assumed
drop size distribution), non-uniform beam filling (NUBF) and
conversion from reflectivity to rainfall intensity (Iguchi et al.,
2009).
By characterizing the rainfall variability within the PR FOV
and as a function of precipitating system types (convective and
stratiform), we will address the impact of the NUBF and the
rainfall classification. By comparing rain rates at ground, some
error factors like the attenuation correction will not be assessed.
Wen et al. (2011) addressed this topic by comparing the radar
reflectivity factors at various heights.
2.3.
Comparison samples
Figure 2(a) shows the mean reference rainfall Rref as a function
of the rain fraction RF and the relative NUBF (RNUBF). Because
of the convolution of the rain rate spectrum by the PR antenna
pattern, various types of rain rate spectra result in similar values
Q. J. R. Meteorol. Soc. (2014)
P.-E. Kirstetter et al.
Figure 2. (a) Mean reference rainfall Rref (mm h−1 ) and (b) mean reference rainfall convective volume contribution (CPI, %) as functions of the rain fraction RF and
the relative non-uniform beam filling (RNUBF).
of Rref . As an example, moderate homogeneous precipitation
filling the FOV (e.g. RF = 95% and RNUBF = 0.2) has the
same average reference rainfall as a scene with inhomogeneous
but heavy precipitation (e.g. RF = 55% and RNUBF = 2).
Lighter rain rates are found for lower beam-filling conditions
(RF<60%) with various quantitative variability. The highest
mean rain rates are found for filled FOV and RNUBF within
the interval (0.5–2). Figure 2(b) shows the reference rainfall type
(CPI) as a function of the RF and RNUBF. The highest convective
contributions are consistently found for higher RNUBF values
(the convective rainfall is associated with higher variability) and
the stratiform type rainfall is associated with lower RNUBF values.
Because of the convolution of the rainfall type distribution in
the FOV by the PR antenna pattern, various contributions from
stratiform and convective rainfall within the FOV result in similar
CPI.
3. Rainfall detection
Studying the impact of satellite sensor detection capabilities leads
to a better understanding of the rain rate spectra as seen from
space and of the climate variability of light rain (e.g. Lin and
Hou, 2012). The goal here is to evaluate PR’s rainfall detection
capabilities and the percentage of rain occurrence and rain volume
likely missed for various precipitation conditions. Lin and Hou
(2012) evaluated the impact of missing light rain due to the PR’s
detection capabilities over the continental United States. They
assumed the minimum detectable rain rate for PR is 0.5 mm h−1 .
Using a product at 4 km horizontal and 1 h temporal resolutions,
they estimated that 43% of total rain occurrence and 7% of total
rain volume is below this threshold. This minimum detectable
rain rate from PR is an approximation, and the capabilities vary
with the horizontal variability within the field of view. Since the
reference rainfall has better sensitivity than PR (Kirstetter et al.,
2012, Figure 6), we focus on cases when the reference is positive.
3.1.
Modelling the rainfall detection by the PR
Satellite rainfall retrievals are often characterized by a rainfall
threshold, under which the detection capabilities degrade (Petty,
1995, 1997; Lin and Hou, 2008, 2012; Berg et al., 2010). It is
commonly noted that PR misses weaker echoes probably due to
its sensitivity (Schumacher and Houze, 2000). We assess here the
threshold detection model because of the significant impact of the
rainfall variability within the FOV on the PR detection capability.
In the ideal case, the distributions of detected and missed reference
rainfall would have no overlap, and the threshold would take on
c 2014 Royal Meteorological Society
a value between the two distributions. In reality, the distributions
overlap.
Figure 3(a) shows the cumulative distributions of rain
occurrence and volume as functions of the rain intensity. Light
precipitation dominates the rain intensity spectra in terms of the
fraction of rain occurrence with 78% of reference rainfall less
than 1 mm h−1 , while the fraction of intermediate rain intensity
(1 mm h−1 < Rref < 10mmh−1 ) is 19%, and the fraction of the
heavy rain intensities (Rref > 10 mmh−1 ) is only 3%. If the PR
detection threshold is 0.5 mm h−1 then it misses 68% of the rain
occurrence. Intermediate and heavy rain intensities dominate
the total rainfall volume. Although heavy rainfall occurs less
frequently than light rain intensities, its contribution is as much
as 58%, with 35% for intermediate rain intensities and only 7%
for light rain intensities. Figure 3(a) depicts the behaviour of the
probability of detection (POD) of the PR as a function of the
reference rain rate. The POD improves with rain rate with a sharp
increase between 0.1 and 1.5 mm h−1 . Yet the POD is neither
null for low rain rates nor equal to unity for high rain rates,
and the transition between the two sills is not a step function.
In finding a reference rainfall threshold defining the detection
capabilities of the PR, there is a trade-off between increasing
the percentage of correct detections and minimizing misses. We
use the Heidke Skill Score (HSS: Wolff and Fisher, 2009; Wilks,
2011) to quantify the accuracy of precipitation detection relative
to random chance at a given rainfall rate. Maximizing the HSS
enables us to identify the threshold and to evaluate this model: 1
indicates a perfect delineation between PR detection above some
threshold and missed precipitation below it; 0 indicates no skill
(the detected occurrence of precipitation is the same as the PR
without correlation); negative values indicate a model no better
than random guess. The HSS is defined as (Brier and Allen, 1951):
HSS =
f2
+
m2
2(hc − fm)
,
+ 2hc + (f + m)(h + c)
(2)
where h, m, f and c are the hits, misses, false detections and correct
rejections (as defined in Kirstetter et al., 2012).
Figure 3(b) shows the HSS score computed for various rain
rate thresholds over the entire dataset. The maximum occurs at
0.53 mm h−1 , a value very close to the 0.5 mm h−1 threshold
assumed by Lin and Hou (2012). Using this value we evaluate the
error as given in the contingency table (Table 1). The difference
between the actual (see Kirstetter et al. (2012) for discussion on
the contingency results) and modelled proportions of detected
rain occurrence is small (2%). However, differences in rainfall
intensities are more significant with an 18% overestimation of the
Q. J. R. Meteorol. Soc. (2014)
Impact of Sub-Pixel Rainfall Variability on TRMM 2A25
Figure 3. (a) Probability of detection of the reference rainfall by the PR ( ) as a function of the reference rain intensity (mm h−1 ). The accumulated fractions of
rain occurrence ( ) and volume ( ) are plotted in grey. (b) Heidke Skill Score for the PR as a function of the reference rain intensity. The dashed line marks the
threshold (0.53 mm h−1 ) identified by the maximum of the HSS score.
Table 1. Contingency table for PR detection of rainfall relative to the reference.
PR
estimates
>0.
=0.
Actual
Model
Actual
Model
Number of
data samples
Percentage
(%)
Mean rainfall
value
Missed
volume (%)
530 261
486 524
1 059 111
1 102 848
33
31
67
69
4.33/4.61
– /5.43
0.0/0.29
– /0.10
–
11.4
4.1
The results are provided for the actual and modelled (section 3) contingency
values. Mean rainfall values associated to the contingency for PR/reference are
provided as well as the proportion of missed volume of rainfall by PR relative to
the reference.
mean reference rainfall value and 65% underestimation of mean
missed reference rainfall. Because the PR consistently detects
higher rain rate and misses lighter rain intensities relative to
the reference (Kirstetter et al., 2012), these discrepancies are
mitigated in terms of total missed volume. Yet, the threshold
model underestimates the missed rainfall volume by 7.3%. These
findings are consistent with those of Lin and Hou (2012), but the
missed occurrence and volume of rainfall are greater here.
The difference between the actual and modelled detected
rainfall suggests that the rain intensity is not the only factor driving
the detection capabilities of PR. The same reference rain rate can
be characterized by various conditions of rain occurrence and
quantitative variability (cf. Figure 2). Hereafter, we quantitatively
examine the characteristics of the reference rainfall at fine spatial
scale within the PR FOV and evaluate how they impact the PR
detection performance.
3.2.
Impact of rainfall variability at fine spatial scale on detection
The impact of rainfall variability within the PR field of view is
assessed with the rain fraction (occurrence variability of rainfall)
and the RNUBF (quantitative variability of rainfall). Kirstetter
et al. (2012) characterized the PR detection performances by using
contingency tables and by simply separating the reference dataset
into ‘robust’ (Rref >σfootprint , i.e. RNUBF<1) and ‘non-robust’
(Rref <σfootprint , i.e. RNUBF>1). We extend this analysis by
separating the reference data into various sub-samples according
to the rain fraction and RNUBF in order to provide in-depth
insight into the PR detection capabilities for various conditions.
All coincident and collocated satellite values are considered and
sorted according to the reference samples. PR sampled robust
rainfall only 33% of the time (80% of total volume of rainfall).
The PR FOV is 80% filled only 26% of the time (84% of total
c 2014 Royal Meteorological Society
volume of rainfall). Situations where the FOV filling is low
and/or the variability of the rainfall is high occur frequently
and require a detailed assessment. Figure 4 shows the behaviour
of the probability of detection (POD) (Figure 4(b)), the mean
missed reference rainfall (Figure 4(c)) and the missed volume
of rainfall (Figure 4(d)) by the PR relative to the reference as a
function of the RF and the RNUBF. Various conditions lead to
equivalent detection performances by the PR. The POD increases:
(i) with higher RNUBF values, and (ii) with high rainfall filling
of the PR pixel. More specifically, the highest POD (>90%) are
found for the conditions (RNUBF>1, RF>90%), and 90% of
rainfall is properly detected for RF>95%. A significant filling
of the FOV (70%) is required to detect rainfall and a high
quantitative variability (RNUBF>1) has a positive impact from
the PR perspective. The detection capabilities degrade significantly
(i.e. POD<50%) for RF<40%. Misses are associated with high
inhomogeneity and/or the ‘rain/no rain’ limits of rain fields.
There are two regimes describing the dependency of the POD on
rainfall variability:
(i) For a given rain filling condition of the FOV, the POD is
mainly driven by the RNUBF for RNUBF values lower than
∼2. As an example with a FOV filled at 70%, only 20%
of the reference rainfall is properly classified for RNUBF
= 0.3 whereas 80% is detected for RNUBF = 2. This rate
remains roughly constant for RNUBF>2, which coincides
with decreasing Rref as functions of RNUBF (see Figure 2)
and does not support the threshold model for detection.
(ii) For RNUBF values greater than ∼2, the POD is mainly
driven by the RF. For such conditions the filling of the FOV
is a significant driver for the detection performances of the
PR.
The missed volume of rainfall is closely related to the POD.
Higher proportions of missed rainfall are noted at lower RNUBF
and RF values, ranging from 10% (RF>80%, RNUBF>0.5) to
70% (RF<20%). The two regimes already noticed for the POD
are more clearly separated for the missed volume of rainfall. The
RNUBF influence is strongly mitigated when values exceed 1. By
comparison, the missed volume passes from 80 to 20% for RNUBF
increasing from 0.2 to 1, respectively. One should note that for all
conditions of RNUBF and RF, the mean reference values are lower
when PR missed them than when they were detected (compare
Figures 2(a) and 4(c)). Both present the same patterns according
to the RF and the RNUBF with higher values for higher rain
fraction and present a maximum around RNUBF = 1.
To evaluate the threshold model (section 3.1), Figure 4(e)
shows the plots of the identified (maximum) HSS values and the
Q. J. R. Meteorol. Soc. (2014)
P.-E. Kirstetter et al.
Figure 4. (a) Number of data samples, (b) probability (%) of rain detection by the PR, (c) mean missed reference rainfall (mm h−1 ), (d) missed volume of rainfall
(%), (e) Heidke Skill Score and (f) corresponding detection threshold of the PR (mm h−1 ) as functions of the rain fraction RF (%) and the RNUBF.
corresponding reference rainfall threshold value as functions of
the RF and the RNUBF. The HSS values are generally greater than
0, showing the threshold model is better than simple random
guess. Yet the accuracy of the threshold model seems to vary with
the rainfall variability characteristics within the FOV. The highest
scores are found for RF values close to the maximum and for lower
RNUBF values; the threshold model is consistent when the PR
c 2014 Royal Meteorological Society
FOV is uniformly filled by rainfall (both according to occurrence
and quantitative aspects). The lowest scores are found when the
FOV is nearly empty and the quantitative variability is high. It is
worth noticing that accuracy of the threshold model quickly drops
for higher RNUBF whatever the RF value. This is particularly true
around RF = 100% and RNUBF = 1, which corresponds to
the highest mean reference rainfall values (see Figure 2). The
Q. J. R. Meteorol. Soc. (2014)
Impact of Sub-Pixel Rainfall Variability on TRMM 2A25
identified detection thresholds are also structured according to
the RF and RNUBF. For RF<60%, detection threshold values
range from 0.2 to 0.6 mm h−1 . These low values are expected
considering the smoothing effect of the antenna pattern on an
FOV partially filled. For RF>60%, detection threshold values
range from 0.5 to 0.8 mm h−1 . These detection thresholds
are higher than the average value 0.53 mm h−1 for conditions
associated with significant contribution to the total volume of
rainfall. This explains why the unique threshold model discussed
in the previous section underestimates the missed rainfall volume.
The rainfall variability at fine spatial scale within the PR FOV
impacts the PR detection performance.
4. Rainfall classification
Precipitation classifications from satellite algorithms have
profound impacts on the accuracy of the quantitative retrievals.
Stratiform and convective clouds have significantly different
vertical heating and moistening profiles. The classification drives
the vertical model of rainfall used to correct for the attenuation
of the radar signal, to estimate the vertical profile of reflectivity,
and the rainfall rate at ground (Iguchi et al., 2009). Classification
of precipitation type from non-polarimetric active remote sensing
relies partly on subjective analysis based on interpretation of
precipitation spatial variability. We investigate relevant factors
(occurrence, type and rate of rain within the PR FOV) driving the
convergence or divergence of the PR classification relative to the
reference. PR’s rainfall classification capabilities, the conditions
driving potential misclassification, and the proportion of rain
likely to be misclassified have been largely unknown. We address
these questions focusing on cases when both the PR and the
reference rainfall are positive, so that precipitation detectability is
not a factor.
4.1.
Convective detection
The PR classification is a binary decision (i.e. either convective
or stratiform), while the ground reference provides a volume
contribution of convective rainfall varying continuously from 0%
(purely stratiform reference rainfall) to 100% (purely convective
reference rainfall). In a similar way to the rainfall detection, we
separated the reference rainfall into purely stratiform (59% of
the data sample) and convective when the convective volume
contribution is positive. Table 2 shows the contingency table for
the PR classification relative to the reference with percentile of
hits (h: both reference and PR detect convective activity), misses
(m: 2A25 classifies as stratiform while the reference classifies as
convective), false alarms (f : 2A25 classifies as convective while
the reference does not), and correct rejection (c: both 2A25 and
the reference classifies as stratiform). Again, all coincident and
collocated PR values are considered and sorted according to the
reference sample. As the false detections (m + f ) have a rate
of 24%, it can be concluded that the PR classification generally
agrees with the reference. The misses are the main contributors to
this population (i.e. 70% of this population). Table 3 provides the
mean rainfall values and the classified rainfall volumes according
to the same contingency tables with PR on the left-hand side of the
‘/’ sign and the reference on the right-hand side. As we expect, the
convective rain rates are higher than the stratiform ones for both
PR and reference. The convective rainfall missed by PR relative
to the reference is consistently associated with lower PR (and
reference) rain rates than when properly classified. Regarding the
rain volumes in question, one can say that PR and the reference
conjointly detect convective rainfall volume (90% for PR, 80%
for the reference). The largest discrepancies are noted for the
conjoint stratiform detection with the PR properly classifying
echoes nearly 90% of the stratiform reference rainfall volume, but
underestimating the volume in question by ∼26%. Nearly 40%
of PR’s stratiform rainfall volume is classified as convective by
the reference. The discrepancies in rainfall volume are much less
c 2014 Royal Meteorological Society
Table 2. Contingency table for PR rainfall classification relative to the reference.
PR estimates
Reference
Convective
Stratiform
Number of data
samples
24%
16.8%
215 494
7.2%
52%
312 901
164 960
363 435
528 395
Convective
Stratiform
Number of data samples
Table 3. Contingency table for PR classification of rainfall relative to the reference.
PR classification/
reference
C/C
S/C
C/S
S/S
Actual
Model
Actual
Model
Actual
Model
Actual
Model
Number of Percentage Mean rainfall (Mis)classified
data samples
(%)
value (mm h−1 ) volume (%)
126 886
175 394
88 608
40 100
38 074
0
274 827
312 901
24
33.2
16.8
7.6
7.2
0
52
59.2
10.8/12.1
10.6/10.6
3.4/4.4
1.9/1.9
4.1/1.8
–
1.7/1.6
1.6/1.6
90/80
100/96.1
39/20
12.9/3.9
10/13
–
61/87
86.9/100
The results are provided for the actual and modelled (section 4) classification
contingency. Mean rainfall values associated to the contingency for PR/reference
are provided as well as the proportion of classified volume of rainfall by PR relative
to the reference.
significant regarding convective reference classified as stratiform
by the PR.
Figure 5 provides an in-depth view of the 2A25 classification by
showing the proportion of convective classification as a function
of the variability within the PR field of view (rain fraction and
RNUBF) for both sensors. The reference classification is regularly
distributed and depends primarily on the RNUBF with values
ranging from ∼3% to more than 95%. This is expected since
the convective rainfall involves more quantitative variability than
the stratiform rainfall. The 2A25 classification shows different
patterns than the reference. There is less dependence on RNUBF,
and the proportion of convective detection does not exceed 85%.
While the classification dependence on the RNUBF shows the
same trend as the reference for RF>50%, the 2A25 classification
shows two detection maxima for extreme RNUBF values in the
domain RF<50%. More investigation is necessary to identify the
reason why the PR is prone to detect convective activity under
these last conditions.
Hits, misses, false alarms and correct rejection for PR’s
detection of convective rainfall are computed and plotted as
functions of the rainfall variability within the PR field of view in
Figure 5. Both products classify convective rainfall consistently
at higher RNUBF values; stratiform classification by both sensors
typically occurs at lower RNUBF values. Misses are not as
prevalent but tend to occur at higher RNUBF values. False
alarms are mainly associated with low filling of the FOV and
low RNUBF values. To summarize, the 2A25 classification differs
from the reference mainly for extreme rainfall variability (i.e. low
filling of the FOV and high quantitative variability). Because the
2A23 classification algorithm is based on a characterization of
horizontal and vertical variability of reflectivity, these cases are
certainly difficult to resolve from the PR perspective.
Grey lines in Figure 6(a) show the cumulative distributions of
CPI occurrence and according convective volume contribution
as functions of the CPI. The CPI is quite evenly distributed
with light convective contribution (CPI<25%) representing
30% of the population, the fraction of intermediate convective
contribution (25%<CPI<75%) being ∼30%, and the fraction
of the significantly convective reference rainfall (CPI>75%)
is 40%. The last class dominates the total convective rainfall
volume with a contribution of more than 90%. Figure 6(a)
also depicts the probability of detection (POD) of the PR as a
Q. J. R. Meteorol. Soc. (2014)
P.-E. Kirstetter et al.
Figure 5. (a) Proportion (%) of convective detection by the reference and (b) by the PR, (c) proportion of conjoint convective classification by the PR and
reference, (d) PR-convective and reference-stratiform classification, (e) PR-stratiform and reference-convective classification and (f) conjoint stratiform classification
as functions of the rain fraction RF (%) and the RNUBF. The colour table is the same for (a,b) and for (c–f).
function of the convective volume contribution to the reference
rainfall. The POD improves with CPI with a sharp increase
from 13% to nearly 40% at light CPI values, then increases
more slowly yet regularly with the convective contribution
to reach ∼80% for a purely convective reference rainfall.
The PR shows consistent classification performances relative
to the reference. Yet the POD is neither null for light CPI
nor equal to unity for high CPI values, and the transition
c 2014 Royal Meteorological Society
between the two sills is not a step function as would be
the case for a perfect classification of rainfall relative to the
reference. In finding a reference rainfall threshold defining
the classification capabilities of the PR, there is a trade-off
between increasing the percentage of convective and stratiform
classification. Similarly with the detection aspect, we evaluate
hereafter the convective contribution threshold describing the
stratiform/convective separation by the PR.
Q. J. R. Meteorol. Soc. (2014)
Impact of Sub-Pixel Rainfall Variability on TRMM 2A25
Figure 6. (a) Proportion (%) of convective detection by the PR as a function of the reference rainfall convective volume contribution (%). The proportion of
convective contribution by occurrence (
) and volume (
) are figured in grey. (b) Heidke Skill Score and (c) corresponding classification threshold (%) of
the PR as functions of the rain fraction RF (%) and the relative non-uniform beam filling RNUBF.
4.2.
Modelling the rainfall classification by the PR
We use the Heidke Skill Score to quantify the accuracy of
precipitation classification (stratiform vs. convective) relative
to random chance at a given reference convective contribution.
By computing the HSS score for various CPI thresholds over the
entire dataset, a maximum value (HSS = 0.56) is found for CPI
= 6%; this indicates the 2A25 product presents overall agreement
with the independent classification provided by the reference.
Assuming this value, we evaluate the discrepancies with the actual
classification as given in Table 3. The difference between the actual
and modelled proportions of classified rainfall underestimates
the wrong classifications (m + f ) by 16.4%, especially the misses.
While presenting overall similar features, differences in rainfall
intensities are also significant especially in case of misses (1.9 mm
h−1 for the modelled convective reference rate misclassified as
stratiform by the PR while 4.4 mm h−1 is the actual value).
The model is close to the actual values when it comes to the
conjoint stratiform classification with values close to 1.6 mm h−1 .
The model overestimates the conjoint convective and stratiform
classification volumes and underestimates the rainfall volumes
implied in misses.
The difference between the actual and modelled classified
rainfall suggests that the CPI is not the only factor driving the detection capabilities of the PR. Hereafter we quantitatively examine the characteristics of the reference rainfall at fine space scale within the PR FOV (cf. Figures 2
c 2014 Royal Meteorological Society
and 3) and evaluate how they impact the PR classification
performances.
Figure 6(b,c) show plots of the maximum HSS and of the
corresponding optimum CPI threshold value as functions of
the RF and the RNUBF. The positive HSS values indicate that
the threshold model is better than random guess, but its accuracy
varies with the rainfall variability characteristics within the FOV.
The highest scores are found for the highest RF and lowest RNUBF
values: this model is consistently all the more accurate when the
PR FOV is uniformly filled by rainfall (both occurrence and
quantitative aspects), which are better conditions for applying the
PR classification algorithm. The model accuracy quickly drops
for higher quantitative variability and RF>50%. This confirms
that the characterization of horizontal variability of reflectivity
is difficult at scales finer than the FOV. The lowest scores are
found when the FOV is nearly empty. That is consistent with
the convective false alarms noticed previously. It seems the limits
of the classification capabilities of the PR are reached for these
conditions.
The identified classification thresholds are structured according
to the RF and RNUBF. Lower threshold values (<10%) are
coincident with RF<80% and higher values ranging from 20 to
90% are found for RF>80%. It is probably easier to detect
convective activity in case of isolated convective cells. The
contribution is more difficult to distinguish when embedded
into stratiform rainfall type. This is confirmed when considering
the dependence of the threshold on the RNUBF. For RF>80%
Q. J. R. Meteorol. Soc. (2014)
P.-E. Kirstetter et al.
Figure 7. (a–d) Reference (left) and PR (right) rainfall rate distributions (mm h−1 ) as functions of the reference rainfall volume convective contribution (%) for the
PR stratiform (top) and convective (second row) classifications. The thick black line represents the median (50% quantile), the dark grey-shaded region represents
the area between the 25 and 75% quantiles, the light grey-shaded region represents the area between the 10 and 90% quantiles. (e,f) Conditional bias of the PR relative
to the reference as a function of the reference rainfall volume convective contribution for the PR (e) stratiform and (f) convective classifications. The grey lines figure
the conditional mean reference (plain) and PR (dotted) rainfall.
(<80%), the threshold values are higher for lower (higher)
RNUBF.
rainfall are positive, so as to remove any discrepancies related to
detectability.
5. Rainfall quantification
5.1.
The last section addresses the dependencies of the PR rainfall
rates on rainfall variability and classification. We address these
questions focusing on cases when both the PR and the reference
The 2A25 algorithm uses different Z –R relationships in the
convective/stratiform profiling components. Figure 7 shows
the reference and PR rainfall rate distributions as functions
c 2014 Royal Meteorological Society
Influence of the rainfall classification
Q. J. R. Meteorol. Soc. (2014)
Impact of Sub-Pixel Rainfall Variability on TRMM 2A25
Figure 8. (a) Mean reference, (b) PR rainfalls (mm h−1 ) and (c) bias (%) of the PR relative to the reference as functions of the rain fraction RF (%) and the RNUBF.
The colour table is the same for (a,b).
of the convective contribution, CPI. All coincident and collocated
PR values are considered and sorted according to the reference
sample. The dataset is also separated according to the PR rain type
classification. Figure 7(a–d) show a shift toward higher rainfall
rates as CPI increases, as we would expect. This shift is most
pronounced for PR and reference rain rates with PR-indicated
convective echoes (Figure 7(c,d)). Despite these consistencies, we
note rain rate distributions indicating higher rainfall rates for the
reference compared to those of PR (i.e. Figure 7(a) compared
to (b), and (c) compared to (d)). The dynamic ranges of rain
rate distribution are greater for the reference than for the PR,
especially for the PR convective type (see Figure 7(c)). Such
differences, which will undoubtedly result in some bias, could be
related to the 2A25 Z –R relationships. It is worth noting that the
spread of the PR convective rain rate distribution in Figure 7(d) is
greater than for the stratiform PR rainfall whatever the convective
contribution. Such a feature does not apply to the reference
rainfall, and potentially indicates uncertainties in quantifying the
2A25 convective rainfall.
The rain type impacts the bias of the PR relative to the
reference. Figure 7(e,f) show the mean rainfall rates and the PR
biases as a function of the CPI. The conditional biases are very
distinct according to the PR rainfall types. The PR convective
systematic biases present a shift towards higher values compared
to stratiform biases. Biases cover a much broader range for the
PR convective type (from −50 to 200%) than for the stratiform
type (from −50 to 10%). They are both decreasing functions of
c 2014 Royal Meteorological Society
the convective contribution CPI, with overestimation at values
<20% and underestimation for convective contribution >90%.
Apparently, both stratiform and convective profiling algorithms
in 2A25 lack sufficient dynamics to deal with extreme rainfall
amounts. However, the behaviour of the bias for convective and
stratiform PR types decreases at a quasi-linear rate with increasing
CPI, and thus provides opportunities for correction using the
ground reference. Biases are consistently distributed with the PR
classification, with the stratiform algorithm presenting relatively
limited biases for light CPI values (biases within 10% for
CPI<40%) and the convective algorithm having limited biases
for high CPI values (biases within 50% for CPI>70%).
5.2.
Influence of the rainfall variability at fine scales
Figure 8 provides an in-depth view of the 2A25 rainfall rate
quantification relative to the reference. The mean rainfall values
by both sensors are computed as a function of the variability
within the PR field of view. The reference rain rates are regularly
distributed along the RNUBF and RF with values ranging from
0.01 mm h−1 to more than 10 mm h−1 (Figure 8(a)). Maximum
rainfall rates occur where RF ∼ 100% and RNUBF ∼ 0.5.
The 2A25 rainfall rate quantification plot in Figure 8(b) shows
different patterns than the reference. Lower gradients are noted
with values ranging from 1 to 7 mm h−1 . A maximum is
found for RF ∼ 100% and RNUBF ∼ 1. This shift of the
2A25 maximum toward higher quantitative variability relative
Q. J. R. Meteorol. Soc. (2014)
P.-E. Kirstetter et al.
Figure 9. Bias (%) of the PR relative to the reference as a function of the rain fraction RF (%) and the relative non-uniform beam filling (RNUBF) for conjoint convective
classification by (a) PR and the reference, (b) PR-convective and reference-stratiform classifications, (c) PR-stratiform and reference-convective classifications, and
(d) stratiform classifications by both PR and reference. The colour table is the same for all panels.
to the reference may result from the NUBF correction scheme
applied in version 7 of the 2A25 algorithm. The bias of the PR
relative to the reference in Figure 8(c) is organized with highest
underestimation (−30%) around RF ∼ 100% and RNUBF ∼ 0.5.
More generally, the PR underestimates relative to the reference
for RF>80% and RNUBF<1. For lower FOV-filling conditions
and higher quantitative variability, 2A25 overestimates relative
to the reference with bias values exceeding 1000%. In these
conditions the rain rates are low and the NUBF correction is
almost non-existent in the 2A25 algorithm.
The bias plot in Figure 8(c) is broken down into contingency
categories for convective classification in Figure 9. When both
reference and PR detect convection, the bias primarily depends
on the RNUBF as depicted with near-horizontal contour lines
of the bias (Figure 9(a)). There is overestimation by PR relative
to the reference for RNUBF>1 and underestimation otherwise.
This feature is more pronounced when PR misses convection, for
which the RNUBF value separating over- and underestimation is
∼2. The 2A25 product overestimates for nearly all conditions of
rainfall variability in cases of falsely detected convection relative
to the reference (Figure 9(b)). When both the reference and
PR classify stratiform rainfall, PR underestimates rainfall rates
relative to the reference only for high filling conditions of the
FOV (RF>90%) (Figure 9(d)). The bias gradients are organized
along the RNUBF axis for lower filling conditions of the FOV
and along the RF axis for higher filling conditions. In convective
rainfall, the quantitative variability inside the PR FOV plays
a significant role in addition to the Z –R relationship (section
5.1).
c 2014 Royal Meteorological Society
6. Conclusions
Satellite surface rain rate estimates are affected by rainfall
variability at finer scales than those resolved by space sensors
and the retrieval algorithms in terms of detection capabilities,
characterization of rainfall types, and quantification of rainfall
rates. A 7-month data sample of TRMM-PR-based rainfall
products was analysed using gauge-adjusted and quality-filtered
surface rainfall estimates derived from NMQ/Q2. Several highresolution Q2 products were used to characterize the reference
rainfall in terms of occurrence, types and rate at PR’s pixel
resolution to evaluate the PR detection, classification and
quantification performances. Primary errors due to incorrect
physical assumptions related to convective versus stratiform
rainfall classification, non-uniform beam filling (NUBF) and
conversion from reflectivity to rainfall intensity have been
investigated. While the error structure of the PR is complicated
because of the interaction of these factors, simple empirical
threshold models regarding PR detection and rainfall classification
were discussed.
Segregating rain from no-rain transition is a driving
contributor to the PR rain rate errors, probably linked to the
lack of sensitivity in the most inhomogeneous and light parts of
the edges of rainy regions, although the PR captures the major part
of the rainfall volume. Rainfall detection capabilities vary with the
horizontal variability within the field of view (non-uniform beam
filling). The PR detects rainfall when the rain amount is high
enough (0.53 mm h−1 ) and the FOV is significantly filled with
rainfall (at least 70%). By utilizing reference rainfall rates at scales
Q. J. R. Meteorol. Soc. (2014)
Impact of Sub-Pixel Rainfall Variability on TRMM 2A25
below the pixel resolution of PR, we have determined that simple
rain rate threshold-based detection models are not accurate in
case of high rainfall variability, and caution is recommended when
using them for evaluating the PR detection performances. The PR
classification generally agrees with the reference. Misclassification
may have a huge impact on the estimated rainfall volume,
with nearly 40% of PR’s stratiform rainfall volume classified
as convective by the reference. The PR classification is generally
consistent with the rainfall quantitative variability within the FOV.
However, misclassification is shown to occur with variability of the
rainfall within the FOV not resolved by the 2A23 algorithm, with
false convective detection associated with low filling of the FOV
and low RNUBF values. Regarding quantification, significant
error is most likely due to a combination of inaccurate Z –R
relationship, non-uniform beam filling and/or attenuation of
the PR radar signal. Both stratiform and convective profiling
algorithms in 2A25 seem to be lacking sufficient dynamics to deal
with extreme rainfall amounts. However, the bias for convective
and stratiform PR types decreases at a quasi-linear rate with
increasing CPI, and thus provides opportunities for correction
using the ground reference. For correcting PR stratiform and
convective rainfall rates, we suggest matching the PR PDFs in
Figure 7(b,d) with those associated to the reference values in
Figure 7(a,c), respectively. For lower FOV-filling conditions and
higher quantitative variability, 2A25 overestimates relative to
the reference and underestimates otherwise. Results from the
conditioned error features presented herein provide insights into
the most significant characteristics of PR rainfall retrieval errors
that need to be taken into account when such data are used in
applications.
Future works will address the relative contributions of errors
linked to off-nadir angle and the underlying terrain. The same
framework and reference rainfall datasets can be readily applied to
rainfall retrievals from other sensors on board low Earth-orbiting
satellites (i.e. TMI, AMSR-E, SSMI, MADRAS). This framework
will also be applied to GPM rainfall estimates following its launch
in 2014. Another important issue to study is how the various error
sources in PR propagate in a number of satellite-based, highresolution precipitation products when calibrating geostationary
infrared-based precipitation estimates.
Acknowledgements
We are very much indebted to the team responsible for the
NMQ/Q2 products, especially Carrie Langston. We want to thank
three anonymous reviewers, whose comments were very useful
in improving the manuscript. The 2A23 and 2A12 products
were obtained from the Goddard Earth Sciences Data and
Information Services Center. This work was funded by a postdoctoral grant from the NASA Global Precipitation Measurement
mission Ground Validation Management.
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